This invention relates to a method for calibrating a magnetic sensor which is designed for checking value documents. The invention also relates to a magnetic sensor that is calibratable by the method according to the invention, and to a value-document processing apparatus having such a magnetic sensor.
For checking value documents there are usually employed sensors for determining the type of the value documents and/or for checking the value documents for authenticity and/or their state. Such sensors are employed e.g. for checking bank notes, checks, identity cards, tickets, vouchers and the like. The value documents are checked in an apparatus for value-document processing which contains one or more different sensors, depending on the value-document properties to be checked.
For example, the value documents are checked using magnetic sensors. Usually, the magnetic sensors are tested for their functionality at certain time intervals or as the need arises, as known e.g. from WO 2010/031576 A1. For testing a magnetic sensor, the latter is first calibrated and subsequently adjusted, if necessary. Calibration is usually effected using calibration media which are fed to the sensor and from which the magnetic sensor senses calibration measurement values. The calibration media can be designed for testing one or more properties of an individual sensor, or for testing a plurality of or all the relevant properties of a plurality of or all the relevant sensors of the apparatus for value-document processing. For example, for calibrating magnetic sensors, paper sheets with known, predefined properties are used as calibration media.
For the value-document check, the value documents to be checked are transported past the magnetic sensor while the latter detects magnetic signals of the respective value document. To calibrate the magnetic sensor, a calibration medium is transported past the magnetic sensor, instead of the value documents, with the magnetic sensor detecting calibration measurement values of the calibration medium. The calibration measurement values are compared with target values which are associated with the calibration medium. If the calibration measurement values of the calibration medium deviate from the target values of the calibration medium, an adjustment of the relevant magnetic sensor is carried out, where applicable, upon which adjustment the sensor is, if possible, so set that it delivers at least approximately the target values when detecting the calibration medium. The thus adjusted sensor is subsequently employed for checking value documents. It is disadvantageous in the hitherto used method that a value document's magnetic signal detected by a magnetic sensor can nevertheless exhibit measurement deviations after calibration of the magnetic sensor, in spite of the subsequent adjustment of the magnetic sensor.